The present invention relates to a carbon dioxide gas sensor for measuring and controlling carbon dioxide gas concentration which is used in monitors for monitoring pollution of the air in living spaces, such as inside a room and inside a vehicle, monitors for controlling air conditioning systems, monitors for managing disaster prevention, monitors for controlling carbon dioxide gas concentration during the transportation and storage of perishable foods, and carbon dioxide gas concentration monitoring system in biotechnical facilities and horticulture under structure, to a process for producing the sensor, and to a carbon dioxide gas sensor device.
Previously known carbon dioxide gas sensors employed for these uses include those of infrared absorption type, electromotive force detection type and capacitance detection type.
An infrared absorption type carbon dioxide gas sensor consists essentially of a light emitting element which emits laser light and a light receiving element sensor which receives the laser light emitted from the light emitting element and measures its intensity. Its working principle lies in receiving the laser light whose intensity has been changed by the presence of carbon dioxide gas and measuring the intensity of the light with the light receiving element sensor, and thereby detecting the carbon dioxide gas concentration. However, though the principle makes use of the fact that carbon dioxide gas absorbs a light of specific wavelength, the light of this wavelength is also absorbed by water, so that the measuring accuracy is adversely influenced by the presence of moisture. Thus, an infrared absorption type carbon dioxide gas sensor, in principle, is capable of accurate determination of carbon dioxide gas concentration if moisture is eliminated; but apparatuses which use this sensor inevitably have a large size and complicated structure owing to the necessity of eliminating moisture, and hence have problems in productivity, mass productivity and production cost, so that the use of sensors of this type has been limited to such special fields as in analytical instruments.
An electromotive force detection type carbon dioxide gas sensor consists essentially of such a solid electrolyte material as NASICON (Na Super Ionic Conductor) and Li-Ti-oxide, and a secondary electrode material comprising alkali metal carbonates, such as Li.sub.2 CO.sub.3 and Na.sub.2 CO.sub.3. In this sensor, alkali metal ions formed by the decomposition of the alkali metal carbonate in the secondary electrode transfer through the solid electrolyte to generate an electromotive force corresponding to carbon dioxide gas concentration. However, NASICON of the solid electrolyte material and the alkali metal carbonate of the main component of the secondary electrode material are liable to dissolve in water. Accordingly, the sensor of this type has problems in that, when used in an environment of high humidity, it shows low reliability and short life due to deterioration of characteristic properties caused by swelling, development of peeling of the electrode, or dissolving out of the alkali metal carbonate.
The above-mentioned problems are overcome to some extent in a capacitance detection type carbon dioxide gas sensor. This sensor makes use of a reversible carbonate-forming reaction between a non-compound type metal oxide and carbon dioxide gas to change its electric property, such as capacitance or impedance and thereby to detect the carbon dioxide gas concentration.
JP-A-4-24548 discloses carbon dioxide gas sensors which detect carbon dioxide gas concentration by changing the capacitance of a mixture of a non-compound type metal oxide, such as CuO and NiO, and a perovskite type oxide, such as BaTiO.sub.3 and SrTiO.sub.3.
These sensors are prepared by using a commercially available material, such as CuO and BaTiO.sub.3, or a material prepared by mixing the commercially available materials and then heat-treating the mixture. The materials used are mixed so as to give a predetermined mixing ratio, then pulverized, shaped and heat-treated to give a plate-formed carbon dioxide gas detection past main body. Thereafter, an electrode part for taking out electric signals is provided to measure the change of such electric properties as capacitance or impedance, thus to complete a carbon dioxide gas sensor.
However, the capacitance detection type carbon dioxide gas sensors as mentioned above which comprise a mixture of a non-compound type metal oxide and a perovskite type oxide or a mixture of an insulating oxide and an oxide which forms a carbonate at high temperature have problems of a low detection sensitivity to carbon dioxide gas and a low speed of response. Moreover, they are still unsatisfactory in moisture resistance and in reliability, for example, they show an insufficient life in long-term operation.
Furthermore, in the capacitance detection type carbon dioxide gas sensor, the necessary change in electric properties is obtained, as mentioned above, by a reversible carbonate-forming reaction between the non-compound type metal oxide and carbon dioxide gas, so that the sensor needs to be heated all the time at a temperature of 400-600.degree. C. Therefore, in a capacitance detection type carbon dioxide gas sensor, which has a heating means as one of the essential constituents, a structure of carbon dioxide gas sensor device which is as small-sized as possible and yet has a good heat efficiency is eagerly desired in order to attain the protection of electric circuits and energy-saving of the heating means itself.